1. Field of the Invention
The present invention relates to a limiter circuit for clipping a white peak in video signals (picture signals) and preventing an occurrence of blooming on the display surface of Cathode Ray Tube (CRT).
2. Background Art
A conventional limiter circuit for clipping a white peak in video signals is shown in FIG. 1. In this Figure, video signals Y (brightness signals) inputted into the input 4 are amplified in the amplifier 1. The video signals (picture signals) are applied to a base of a transistor Q.sub.1, which is one of a pair of differential transistors Q.sub.1 and Q.sub.2. A reference voltage E is applied to a base of the differential transistor Q.sub.2. A fixed voltage V.sub.cc is applied to the collectors of both the transistors Q.sub.1 and Q.sub.2. In accordance with the amplitude of the picture signals Y, the collector current and the emitter current of the transistor Q.sub.1 changes. Therefore, a voltage V.sub.23 between point 2 and point 3 changes due to a voltage drop on the resistance R.sub.1. The voltage V.sub.23 is outputted at an output 5.
In this construction, it is assumed that video signals Y inputted into the input 4 and amplified by the amplifier 1 are as shown in FIG. 2A. In FIG. 2A, reference numeral 11 indicates a black level, 12 indicates a white level and 13 indicates a horizontal synchronizing signal. When the amplitude of the picture signal Y comes into the vicinity of the white level, the collector current of the transistor Q.sub.1 is cut off, so that the transistor Q.sub.1 is OFF. At this time, the reference voltage E is outputted at the output 5. Therefore, when the picture signals Y shown in FIG. 2A outputted from the amplifier 1 are applied to the base of the transistor Q.sub.1, signals Y' are outputted at the output 5, as shown in FIG. 2B. In FIG. 2B, reference numeral 14 indicates a level of the reference voltage E [v].
As described above, according to the conventional limiter circuit, since a picture signal in the vicinity of the white level is clipped, the occurrence of blooming on the display surface of the CRT is prevented, and therefore, the picture quality of the CRT display is improved.
However, the conventional limiter circuit described above has problems, as will be discussed below. In such a conventional limiter circuit, a white peak is always clipped, with no relation to the contrast of the picture on the display surface of the CRT. Therefore, in the case where bright white points are required on the picture, and it is unnecessary to improve the picture quality, bright white points cannot be obtained with the conventional limiter circuit, because of the abovedescribed limitting operation. Particularly, in the case of Projection Television (PJTV), the maximum value of brightness is very important. Therefore, problems are encountered in using a conventional limiter circuit in a Projection-type Television.
Therefore, it is a principal object of the present invention to clip a white peak when a value of contrast of a display device is below a certain predetermined point, but not clip the white peak where the contrast value equals or exceeds the predetermined point. Thus, bright white points may be obtained in the latter case.
Additional objects and advantages will be obvious from the description, or may be learned by practice of the invention.